Site-defined and reproducible bubble nucleation is essential for microscale phase-change heat transfer, laser-induced thermofluidics, and localized thermal management. However, conventional heating methods usually produce broad thermal disturbances and poorly constrained nucleation sites. Here, we demonstrate low-power bubble nucleation in water by exploiting the localized photothermal response of a silicon nanotip under continuous-wave 532 nm laser irradiation. A two-dimensional electromagnetic-thermal coupled finite-element model is first established to resolve optical absorption and steady-state heat conduction around the immersed nanotip. The simulations show that the maximum temperature rise does not occur when the laser focus coincides with the nanotip apex, but appears at an axial offset of approximately D = 2 μm, where the focused beam overlaps more effectively with the absorbing volume of the conical tip. Increasing the apex radius from 6 nm to 8 nm enhances the temperature rise, whereas further increasing the radius to 10 nm leads to near saturation because optical absorption and heat dissipation reach a new balance. Raman thermometry based on the full width at half maximum of the first-order silicon Raman peak gives a temperature-rise coefficient of (7.72±0.36) K/mW for the nanotip, which is about 3.5 times that of a flat silicon substrate, (2.22±0.43) K/mW. Under continuous irradiation at 25 mW, a stable bubble forms at the nanotip apex, corresponding to a local temperature of approximately 485 K. Classical heterogeneous nucleation estimates and thermal-diffusion time-scale analysis indicate that bubble formation is dominated by vapor nucleation in locally superheated water, while the nanoscale apex curvature lowers the effective nucleation barrier beyond the prediction of a planar-interface model. These results clarify the coupling among nanotip geometry, localized photothermal heating, and low-power nucleation threshold, providing a feasible route for low-disturbance microscale phase-change control.